1. Field of the Invention
This invention relates to a method for in-situ treatment of dense non-aqueous phase liquids so as to immobilize them in the subsurface, thereby reducing the release rates of contaminants of concern disposed therein, such as benzene and naphthalene, and providing environmentally acceptable management of a contaminated site. More particularly, this invention relates to a method for in-situ solidification of dense non-aqueous phase liquids in contaminated subsurface sites and for removal of rapidly released contaminants from such contaminated subsurface sites.
2. Description of Related Art
In the United States alone, there are thousands of sites that are contaminated with a type of dense non-aqueous phase liquids (DNAPL) comprising a mixture of components that have different physical properties and that behave differently from one another at ambient and elevated temperatures. One example of this type of contamination is the tar dense non-aqueous phase liquids generated from coal processing, such as occurred in manufactured gas plants (MGP) or in coking in the steel industry. This special type of dense non-aqueous phase liquids is mainly comprised of a heavy fraction that exists as an immobile solid at ambient temperatures and a minor fraction of light hydrocarbons (with low molecular weight and high vapor pressure), e.g. benzene and naphthalene, that exist as liquids at ambient temperatures. The compounds of the minor fraction operate to solubilize the heavy fraction when the two fractions are combined. Although the minor “solubilizing fraction” of organic compounds may comprise as little as 3% of the weight of the dense non-aqueous phase liquids, its presence determines the rheology, physical flow and mobility, partitioning of contaminants into water and gases, and rates of contaminant release to groundwater.
Contaminants of concern from MGP sites can be grouped in the following three categories:                1) Benzene, Toluene, Ethylbenzene, Xylene (BTEX)        2) Polyaromatic Hydrocarbons (PAHs)        3) Total Petroleum Hydrocarbons (TPHs).A certain maximum contaminant level (MCL) has been established for volatile contaminants in drinking water. For example, the allowable MCL for benzene is 0.005 mg/l. This value of benzene for drinking water can be considered as very stringent regulations. However, for cleanup criteria applied to some areas of MGP sites, the non-hazardous levels are sufficient for remediation of sites. These numbers are often an order of magnitude higher than for drinking water. The benzene level for non-hazardous level concentration is 0.5 mg/l. While treating MGP sites containing tar/soil contaminated with BTEX, it is, therefore, important to accomplish the non-hazardous level of the contaminants while implementing a treatment technology.        
Thus, soil/tars must be treated at operating conditions sufficient to accomplish enough removal of benzene so that residual levels are below 0.5 mg/l levels in the TCLP test. No minimum levels are defined for lighter PAHs; however, they should be removed from the soil/tar to render soil/tar immobilized.
In the U.S., there exist more than 3,000 former MGP sites. About 400 of the sites have been characterized and about 125–150 of these have received attention in the form of some type of remediation. Less than 50 sites have been satisfactorily resolved or “closed”. Currently, it is estimated that over the next two decades more than 20–40 sites per year will require immediate solutions for the mitigation of highly concentrated source areas (defined as source areas with more than 1% contamination in the soil); each of these sites will contain at least three distinctive source areas requiring treatment (e.g. gas holders, relief holders, tar separators, soil hot spots). It is expected that a successfully developed in situ thermal treatment technology could be applicable to most of these sites.
At present, state of the art methods for addressing the problems of dense non-aqueous phase liquids involve thermal desorption, which is usually conducted with ex-situ processing, requiring the contaminated soil to be excavated from the subsurface. Thermal desorption typically involves heating of the contaminated soil/DNAPL material in a rotary kiln-type device at temperatures in the range of about 300° C. to about 700° C., vaporizing more than 98% of all of the organics, and disposing of the off-gas stream through combustion or condensation.
Conventional methods of applying thermal desorption to achieve in-situ removal of coal-based DNAPL have been less than highly successful. Attempts to apply classical thermal desorption temperatures in-situ to achieve efficient removal of the total DNAPL mass have been largely unsuccessful due to the inability to reach the required target temperature range of 400°–700° C. using commercial heating hardware. Barriers to reaching these temperatures in the subsurface include: 1) the heat of vaporization due to the high moisture conditions of many sites with shallow water tables, and 2) the loss of heat transfer efficiencies in soils that lose virtually all moisture (which rapidly occurs at temperatures above 200° C.). A further disadvantage of conventional thermal desorption methods is the large outputs of hydrocarbon contaminants resulting from the high temperature desorption that require expensive handling and disposal. These limitations point to the need for a new strategy for the effective use of thermal treatment.